/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;

/**
 * A transformation from one object to another. For example, a
 * {@code StringToIntegerFunction} may implement
 * <code>Function&lt;String,Integer&gt;</code> and transform integers in
 * {@code String} format to {@code Integer} format.
 * 
 * <p>
 * The transformation on the source object does not necessarily result in an
 * object of a different type. For example, a {@code FarenheitToCelsiusFunction}
 * may implement <code>Function&lt;Float,Float&gt;</code>.
 * 
 * <p>
 * Implementations which may cause side effects upon evaluation are strongly
 * encouraged to state this fact clearly in their API documentation.
 * 
 * @param <F>
 *            the type of the function input
 * @param <T>
 *            the type of the function output
 * @author Kevin Bourrillion
 * @author Scott Bonneau
 * @since 2010.01.04 <b>stable</b> (imported from Google Collections Library)
 */
@GwtCompatible
public interface Function<F, T> {

    /**
     * Applies the function to an object of type {@code F}, resulting in an
     * object of type {@code T}. Note that types {@code F} and {@code T} may or
     * may not be the same.
     * 
     * @param from
     *            the source object
     * @return the resulting object
     */
    T apply(@Nullable F from);

    /**
     * Indicates whether some other object is equal to this {@code Function}.
     * This method can return {@code true} <i>only</i> if the specified object
     * is also a {@code Function} and, for every input object {@code o}, it
     * returns exactly the same value. Thus, {@code function1.equals(function2)}
     * implies that either {@code function1.apply(o)} and
     * {@code function2.apply(o)} are both null, or
     * {@code function1.apply(o).equals(function2.apply(o))}.
     * 
     * <p>
     * Note that it is always safe <em>not</em> to override
     * {@link Object#equals}.
     */
    boolean equals(@Nullable Object obj);
}
